In the past, in plants, factories, or the like, distributed control systems (DCSs) in which on-site devices (measurement devices or operation devices) called field devices and a control device controlling the field devices are connected by communication means have been structured to realize advanced automatic operations. Most communication systems which are bases of such distributed control systems perform communication in a wired manner. In recent years, however, communication systems have also been realized that perform communication wirelessly in conformity with a wireless communication standard such as ISA100.11a. Here, the foregoing ISA100.11a is an industrial automation wireless communication standard developed by the International Society of Automation (ISA).
A communication system that conforms to the foregoing wireless communication standard, ISA100.11a, broadly includes a system manager (control station) serving as a control device, a backbone router (master station) connected to the system manager, and a wireless field device (slave station) performing wireless communication with the backbone router. The system manager and the backbone router are connected to an asynchronous communication network called, for example, a backbone network. The backbone router and the wireless field device are connected to a synchronous wireless communication network. The backbone network is realized as a wired communication network in some cases and as a wireless communication network in other cases.
The following Non-Patent Document 1 discloses a communication system in which a time-division multiplexing communication scheme is used as a communication scheme between master stations and slave stations, a plurality of master stations are installed separately (installed redundantly), and the plurality of master stations simultaneously receive signals transmitted from a plurality of slave stations, thereby ensuring a delay time with high reliability. Further, the following Non-Patent Document 2 discloses a method of synchronizing times with high accuracy between devices on a network such as Ethernet (registered trademark) in which a delay time varies considerably. The following Patent Documents 1 and 2 disclose technologies for avoiding fluctuation in transmission delay inherent to a wireless local area network (LAN) in a wireless LAN standard such as IEEE 802.11. Further, the following Patent Document 3 discloses a technology for performing time synchronization directly between a plurality of master stations when the master stations that perform communication in a time-division multiplexing communication scheme coexist.